The present invention is directed to integrated circuits. More particularly, the invention provides distortion reductions to amplification systems. Merely by way of example, the invention has been applied to a Class-D amplifier. But it would be recognized that the invention has a much broader range of applicability.
Usually, a switching amplifier (e.g., a Class-D amplifier) is an amplifier where output transistors are often operated as switches. The Class-D amplifier is widely used in audio amplification, and has power-efficiency advantages over certain linear audio-amplifier classes such as Class A, B, and AB.
FIG. 1 is a simplified conventional diagram showing an amplification system using a Class-D amplifier. The amplification system 100 includes a modulation component 102, a gate driver 104, two transistors 106 and 108, an inductor 110, two capacitors 112 and 114, and an output load 116. For example, the transistor 106 is a P-channel metal-oxide-semiconductor field effect transistor (MOSFET), or an N-channel MOSFET. In another example, the transistor 108 is an N-channel MOSFET. In yet another example, the output load 116 is a speaker. In yet another example, the inductor 110 and the capacitor 112 are included in a low pass filter 130. In yet another example, the modulation component 102, the gate driver 104, and the transistors 106 and 108 are included in a Class-D amplifier 118.
The modulation component 102 receives an input audio signal 120, and generates a modulation signal 122. The gate driver 104 receives the modulation signal 122, and generates in response gate drive signals 124 and 126. The transistors 106 and 108 receive the gate drive signals 124 and 126 respectively, and generate an output voltage signal 128 (e.g., Vout). The low pass filter 130, together with the blocking capacitor 114, receives the output voltage signal 128, and in response generates an output audio signal 132 to drive the output load 116. The output voltage signal 128 (e.g., Vout) is fed back to the modulation component 102. For example, the gate drive signal 124 is equal to the gate drive signal 126. In another example, the output audio signal 132 is proportional to the input audio signal 120 in magnitude. In yet another example, the gate drive signals 124 and 126 are logic control signals, and hence the transistors 106 and 108 operate like switches.
But in some situations, as the input audio signal 120 is processed by the modulation component 102, the gate driver 104 and the transistors 106 and 108, certain distortion is introduced into the output voltage signal 128, and thus the quality of the output audio signal 132 is reduced. Usually, the output voltage signal 128 is fed back to the modulation component 102 in order to reduce the distortion. Additionally, the modulation component 102 often includes a first-order integrator or a higher-order integrator (e.g., a second-order integrator) to reduce the distortion. A higher-order integrator usually has a higher gain than a first-order integrator, and performs better in reducing the distortion.
FIG. 2 is a simplified conventional diagram showing certain components of a second-order integrator as part of the modulation component 102. The second-order integrator 200 includes two first-order integrators 214 and 216 connected in series. The first-order integrator 214 includes an operational amplifier 202, a resistor 206, and a capacitor 210. The integrator 216 includes an operational amplifier 204, a resistor 208, and a capacitor 212. The capacitor 210 is coupled between an output terminal and an input terminal of the amplifier 202, and the capacitor 212 is coupled between an output terminal and an input terminal of the amplifier 204.
An input signal 216 is received at the resistor 206, and a signal 224 is generated in response. The operational amplifier 202 receives the signal 224 at one input terminal and a reference signal 220 at the other input terminal, and generates in response a signal 226. The resistor 208 receives the signal 226, and a signal 228 is generated in response. The operational amplifier 204 receives the signal 228 at one input terminal and a reference signal 222 at the other input terminal, and generates in response a signal 218. For example, the reference signal 222 is equal to the reference signal 220.
For example, a small signal transfer function of the integrator 200 is determined according to the following equation:
                              H          ⁡                      (            s            )                          =                  1                                    s              2                        ⁢                          R              1                        ⁢                          R              2                        ⁢                          C              1                        ⁢                          C              2                                                          (                  Equation          ⁢                                          ⁢          1                )            
where H(s) is the small signal transfer function of the integrator 200, s represents a complex variable of Laplace transform, R1 represents the resistance of the resistor 206, and R2 represents the resistance of the resistor 208. Additionally, C1 represents the capacitance of the capacitor 210, and C2 represents the capacitance of the capacitor 212. According to Equation 1, the transfer function H(s) of the integrator 200 has two poles at which the transfer function H(s) reaches approximately infinity.
FIG. 3 is a simplified conventional diagram showing a Bode plot of the transfer function H(s) of the integrator 200 as part of the modulation component 102. The waveform 302 represents the magnitude of the transfer function H(s) of the integrator 200 as a function of frequency. The waveform 304 represents the phase angle of the transfer function H(s) of the integrator 200 as a function of frequency.
As shown in FIG. 3, the magnitude and the phase angle of the transfer function H(s) of the integrator 200 decrease as the frequency increases. For example, at a frequency 306, the magnitude of the transfer function H(s) of the integrator 200 is 0 dB, and the phase angle of the transfer function H(s) of the integrator 200 is −180°. The phase margin is 0°, and thus the amplifier 118 is often unstable. Hence, zero compensation is usually needed to yield sufficient phase margin in order to keep the amplifier 118 stable.
Furthermore, the saturation of the integrator 200 often causes distortion. FIG. 4 is a simplified conventional timing diagram of the input audio signal 120. The waveform 402 represents the input audio signal 120 as a function of time. For example, the input audio signal 120 has a sinusoidal waveform as shown by the waveform 402, and has a constant period T0. The amplitude of the input audio signal 120 varies periodically over time.
FIG. 5 is a simplified conventional timing diagram of the output audio signal 132 in response to the input audio signal 120 as shown in FIG. 4 for the amplification system 100 that includes the second-order integrator 200. The waveform 502 represents the output audio signal 132 as a function of time. The output audio signal 132 has a period T1 as shown by the waveform 502. For example, the period T1 is approximately the same as the period T0 of the input audio signal 120. The output audio signal 132 generally follows the change of the input audio signal 120 as shown by the waveform 502. But, the output audio signal 132 contains distortions 504 due to the saturation of the integrator 200.
Referring back to FIG. 2, for example, the input signal 216 includes both the input audio signal 120 and the output voltage signal 128 that is fed back to the modulation component 102. In another example, if the input audio signal 120 is not much larger than the output voltage signal 128, the input signal 216 is proportional (e.g., equal) to the input audio signal 120 superimposed with the output signal 128. The second-order integrator 200 receives the input signal 216, and outputs the signal 218 that is within a certain range. But, in yet another example, if the input audio signal 120 is much larger than the output voltage signal 128, the signal 218 output by the integrator 200 is approximately equal to a positive power supply voltage or ground. That is, the integrator 200 becomes saturated. Then, if the input audio signal 120 reduces in magnitude, the response of the system 100 to the change of the input audio signal 120 lags behind due to the saturation of the integrator 200. In yet another example, distortions 504 are hence introduced into the output audio signal 132.
Hence it is highly desirable to improve the techniques of distortion reductions to amplification systems.